1. Field of the Invention
The present invention relates in general to a safety system installed in a wheeled motor vehicle, and more particularly, to a seat belt system for restraining a seat occupant in the motor vehicle.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional seat belt system, which is disclosed in Japanese Utility Model First Provisional Publication No. 1-89256, will be described with reference to FIGS. 10 and 11.
As is shown in FIG. 10, the seat belt system is applied to a front seat 11 of a motor vehicle. The seat belt system comprises generally a lap-shoulder combination belt system 12 and a thigh belt system 31, which functions to safely restrain a seat occupant in an emergency such as a vehicle collision or the like.
The lap-shoulder combination belt system 12 includes a belt 13 which has a shoulder belt part 13a and a lap belt part 13b. In fact, the shoulder belt part 13a and the lap belt part 13b are bounded by a through ring 33 through which the belt 13 passes. The through ring 33 has a tongue which is latchingly engageable with a buckle member mounted at an inboard portion of a vehicle floor. An upper end of the shoulder belt part 13a has an anchor plate 7 fixed to a center pillar 9 of the vehicle, and a lower end portion of the lap belt part 13b is retracted by an emergency locking type belt retractor 32 arranged beside the seat 11.
The thigh belt system 31 comprises a belt 34 which has one end portion retracted by the belt retractor 32 and the other end fixed to the through ring 33. Two belt supporters 35a and 35b are located at both sides of the seat 11 for suspending the thigh belt 31.
When in use, the through ring 33 is engaged with the buckle member and, as is seen from FIG. 11, the thigh belt 34 is hitched round the belt supporters 35a and 35b having a major portion thereof tightly put on the thighs of the seat occupant "P".
Due to its inherent construction, the seat belt system has the following characteristics.
First, for the seat occupant "P", it is troublesome to manually hitch the thigh belt 34 round the belt supporters 35a and 35b.
Second, in a vehicle collision, belt slippage tends to occur. That is, upon a vehicle collision, the seat occupant "P" is moved forward causing instant locking of the retractor 32 and thus the upper half of the seat occupant "P" is inclined forward. The forward but slight inclination of the upper half of the seat occupant "P" applies the lap belt part 13b with a considerable tension "T.sub.1 " leaving the thigh belt 34 unchanged or nontensioned. This phenomenon brings about a considerable moment "M" produced about the center "G" of the hip of the seat occupant "P", which moment is represented by the following equation: EQU M=(T.sub.1 .times.l.sub.1)-(F.times.l.sub.3)&gt;0 (1)
wherein:
T.sub.1 : tension applied to the lap belt part 13b; PA1 l.sub.1 : distance between the center "G" and the lap belt part 13b; PA1 F: counterforce applied by the seat to the hip of the seat occupant "P"; PA1 l.sub.3 : distance between the center "G" and the lower surface of the hip.
Thus, upon a vehicle collision, the hip of the seat occupant "P" is swung but slightly in a clockwise direction in FIG. 11 causing a slippage of the lap belt part 13b from the originally set restraining position.